Sagitta

As autumn approaches, the leaves of many trees turn yellow, orange, and red. At the same time, the night sky features a tiny constellation whose brightest stars are doing the same.

The constellation is Sagitta, the arrow. Although it’s small, it’s lodged in a prominent part of the sky, between Aquila, the eagle, and Cygnus, the swan. It’s high in the southeast as night falls, and shoots over to the west as the evening progresses.

Sagitta’s four brightest stars are all in the autumn of their lives. They were once main-sequence stars like the Sun, so they generated energy by converting hydrogen into helium in their cores. But the stars eventually used up the hydrogen in their cores, so they began to burn hydrogen around their cores. This caused the stars to expand to become giants.

Two of Sagitta’s stars — Alpha and Beta Sagittae — are yellow giants, while two others — Gamma and Delta Sagittae — are red giants. Gamma is the constellation’s brightest star, because it’s the closest of the four, at a distance of about 260 light-years from Earth. The other three stars are 400 to 600 light-years away.

Sagitta also houses an entire cluster of aging stars that’s 13,000 light-years away. Named M71, the cluster’s stars are all about twice as old as the Sun. And its brightest members are yellow and orange giants — all part of a constellation that reflects the changing of the seasons.

Tomorrow: measuring lunar “lumps.”

Script by Ken Croswell, Copyright 2011

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